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Патент USA US3034005

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May 8, 1962
F. L. PUTZRATH
3,033,995
CIRCUIT FOR PRODUCING AN OUTPUT VOLTAGE INDICATIVE OF THE
ABSOLUTE VALUE OF THE DIFFERENCE
BETWEEN Two INPUT VOLTAGES
Filed Aug. 51, 1960
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BY
United States Patent G
1
'
3,033,995
'
HNDECATIVE UP THE ABSQLUTE VALUE OF
TEE DH‘FERENCE BETWEEN TWO INPUT
VGLTAGES
Franz Ludwig Putzrath, Oakiyn, N.J., assignor to Radio
Corporation of America, a corporation of Delaware
Filed Aug. 31, 1960, Ser. No. 53,212
6 Claims. (Cl. 307-885)
Patented May 8, IQGZ
current flows from terminal 24, through resistor 28,
through the ‘base-to-emitter diode of transistor 12, through
resistor 18, through the emitter-to-base diode of transis
tor 10, through resistor 34- to terminal 26. )Since emitter
to-base current ?ows through transistors 10 and 12, col
lector current also ?ows through these transistors and a
given output voltage appears across output terminals 36.
Current does not flow through transistors 14 and 16. The
reason is that the polarity of the input voltages V1 and
This application relates to a new and improved dif
'
ICC
2
cmourr FQR PRODUCING AN OUTPUT VOLTAGE
ference ampli?er.
:
>
‘In some pattern matching equipments such as used in
communication systems, reading machines, shape recog
10 V2 is such ‘as to reverse bias the emitter-to-base diodes of
transistors 14 and 16. —4 volts is applied to the base of
transistor 16 and —2 volts is applied to the base of tran
sistor 14 and the voltage diiference is therefore in a sense
to reverse bias the emitter-to-base diodes of both tran
nition machines, and the like, it is necessary to compare 15 sistors.
two input quantities such as voltages or currents and to
Assume now that the input voltage V1 is -—4 volts and
derive therefrom an output signal which is proportional to
the input voltage V2 is —2 volts. The di?erence in volt
the difference in these quantities. In certain of the above
age between V1 and V2 is now of polarity to forward
applications it is desirable to determine only the absolute
bias the emitter-to-base diodes of transistors 14- and 16
magnitude of the di?erence in quantities regardless of 20 and to reverse bias the emitter-to-base diodes of transis
which of the quantities is larger, and regardless of the
tors 10 and 12. Current now ?ows through transistors 14
absolute value of either of the quantities. Conventional
and 16 and transistors 10 and 12 are cut on‘. The output
di?erence ampli?ers do not meet these requirements as
voltage across terminals 36 is exactly the same ‘as the out
they produce an output having a sense dependent upon
put voltage at these terminals when V2 is —4 volts and
which of the input quantities is the larger.
. 25 V1 is —2 volts.
.
The circuit of the present invention solves the problem
If the voltage V2 should change to a value of -—6 volts
above in a simple, reliable and relatively inexpensive way.
and the voltage V1 to a value of -—4 volts, the absolute
The circuit includes two parallel si@al paths, each com
magnitude of the difference in voltage'is still 2 volts.
prising two transistors of opposite conductivity connected
Under these conditions, the same amount of current flows
in series. An operating voltage is applied to the transis 30 through the emitter-to-base diode of transistors 10 and
tors in both paths through a common load impedance.
12 as when the voltages V2 and V1 are —4 ‘and —2
An input voltage having a magnitude equal to the dif
ference between two input quantities is applied in one
volts, respectively. Accordingly, the voltage V3 at tenni
nals 36 is again the same as in the cases described above.
sense between the bases of the transistors in one path and
When V1 is equal to V2, there is no difference in voltage»
in an opposite sense between the bases of the transistors 35 applied between the bases of any of the transistors. For
in the other path. An output voltage which is propor
example, the same voltage is applied to the base of tran
tional to the di?erence ‘between the two input quantities
sistor 14; as to the ‘base of transistor 16. Accordingly, no
appears at the common load impedance. The sign of
current ?ows in the emitter-to-base diodes of these tran
the output voltage is independent of the sense of the dif
sistors, and since no base current ?ows, no collector cur
ference between the input quantities or the absolute value
of either of the input quantities.
The invention is discussed in greater detail below and
is illustrated in the following drawings of which:
FIG. 1 is a schematic circuit diagram of the present
invention; and
FIGS. 2 and 3 are graphs illustrating the performance
of the circuit of FIG. 1.
rent ?ows. Accordingly, the output voltage V3 at termi
nals 36 is ~25 volts. The same analysis holds for tran
sistors 10 and 12. -No emitter-to-base current ?ows in
either of these transistors and the output voltage remains
at —25 volts as already stated.
The purpose of resistors 18 and 21) is to provide a small
amount of reverse bias on the transistors.
This ensures
that there is essentially no current ?owing through the
load resistor 22 when the two input voltages ‘are identical.
An additional advantage due to resistors 18 [and 20 is that
they provide a large amount of feedback. It is therefore
The diiference ampli?er of FIG. 1 includes two parallel
signal paths, the ?rst including two transistors 10 and 12
of opposite conductivity type connected in series and the
other including two transistors 14 and 16, ‘also of opposite
not necessary to balance transistors, and any units in a
conductivity type, connected in series. Each of the signal
production run may be used.
.
paths includes, in addition to the transistors, resistors 18
A practical circuit according to FIG. 1 may have the
and 20, respectively. An operating voltage, shown as —25
following values of circuit elements:
volts, is ‘applied to the transistors through a common load 55
Resistor-s 28, 30, 32 and 34—10,000 ohms each
impedance, such as a resistor 22.
Resistors 18 and 20~2,700 ohms each
The two input voltages V1 and V2 are concurrently ap
Resistor 22—5,600 ohms
plied between terminals 24 and 26, respectively. One of
PNP transistors 10 and 14-—2 N 404
terminals 24 is connected through resistors 28 and 30' to
the bases of the NPN transistor 12 in one path and the 60 NPN transistors 12 and 16-2 N 440
PNP transistor 14 in the other path, respectively. One
A circuit with the values of components above and the
of terminals 26 is similarly connected through resistors
32 and 34 to the bases of the NPN transistor 16 and
PNP transistor 10, respectively. The other of the termi
operating voltage shown produces outputs as shown in
.FIG. 2. It may be seen that the same output voltage is
produced for a given difference in input voltages, regard
nals 24 and 26 are connected to a point of reference po
65 less of the magnitudes of the input voltages or the sense
tential, shown as ground.
of the difference. This is also shown in a somewhat dif
The operation of the circuit of FIG. 1 may be better
ferent way in FIG. 3 which illustrates that the output
understood by assuming certain input voltages. Assume
voltage depends only on the difference in magnitudes of
?rst that voltage V2 is somewhat more negative than
the input voltages and not on the individual magnitudes of
voltage V1. For example, voltage V2 may be -4 volts
and voltage V1, —2 volts. Under these conditions, the 70 the input voltages or the sense in the difference in the
sense of the voltage difference is such that “conventional”
magnitudes. -
- : 3,033,996
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.What is claimed is:
,1
a
3
4
.
'
' one conductivity type in one circuit and the base of a
'
transistor of opposite conductivity type in the other cir
1. iin combination, two circuits connected in paralle ,
cuit; and means for applying a second input signal to
the bases of the other two transistors.
‘
,l
connected in series; a common load impedance connected
5. A difference ampli?er comprising, in combination,
_ in series with the two circuits through which an operating Cl
a ?rst pair of transistors of opposite conductivity type
voltage for all of said transistors may be applied; means ,
connected in series ,emitter-to-emitter; a second pair of
.t'or applying .‘an input voltage across the bases of the
each comprising two transistors of opposite conductivity
transistorsin one circuit; and means'ior applying the
' same inputjvoltage in an opposite sense across'the bases
of the transistors in the other circuit.
‘
a,
10
- 2. In combination, a ?rst pair of transistors of opposite
conductivity type connected in series and in a sense to
conduct current in the same direction; ‘a second pair of
transistors of opposite conductivity type connected in
series and in a sense to conduct current in the same di
transistors of opposite conductivity type connected in
series emitter-to-emitter; a ?rstvcommon connection at
the collectors of two of'the transistors of like conductivity
type; a second common connection at the collectors of
the other two transistors; a load impedance connected at
one terminal torsaid ?rst common connection for supply
ing an operating voltage’ to the transistorsg‘means for
applying ‘a first input signalbetween said second common
rection, the two pairs of transistors being connected in
connection and the bases of :two transistors, one in each
parallel; a load resistor connected to both pairs of tran
pair, of opposite conductivity type; and means for apply
sis'tors through which an operating voltage for all of said
transistors may be applied; and means for supplying a
diiference voltage across the bases ‘of one pair of tran:
sistors ‘and for supplying the same voltage in opposite
polarity across the bases of the other pair of transistors“
3. ‘In combination, two circuits connected in parallel,
ing a second input signal between said second common
connection and the bases of the other two transistors, one
each comprising two transistors of opposite conductivity
similarly connected in series, like electrode to like elec
trode; acommon load resistor connected in series with
the two parallel circuits for applying an operating volt
pair of opposite conductivity'type;
270 in 6.,each
A di?ference ampli?er comprising, in combination,
a ?rst pair of transistors of opposite ‘conductivity type
connected in series emit-ter-to-emitter; a second pair of
‘transistors of opposite conductivity type connected in
series emitter~to~emitter; two resistors of like value, each
connected in seriespwith a pair of transistors between
' emitters; a ?rst common connection at ‘the collectors of
two of the transistors of like conductivity type; a second
age to said transistors; and means for applying an input I
voltage in one sense between the basesof the transistors " common connection at the collectors of the other two
in one circuit‘and'in an opposite sense between, the bases so transistors; a load impedance connected at one terminal
of the transistorsin the other circuit. 7
'
'
‘to said ?rst common connection for supplying anoperat
4. 'In combination,_rtwo parallel circuits, each compris-
ing voltage to the transistors‘, means for applying a ?rst
series emitter-to—emi-tter; a two, terminal common load '
the bases of two transistors, one in each pair, of opposite
conductivity type; and means for applying a second input
signal between said‘ second common connection and the
,ing two transistors of opposite conductivity connected in ' 1 input signal between said second common connection and
resistor for ‘the two circuits connected at one terminal
to the collectors of the two transistors of one conductivity
type, whereby an ‘operating voltage may be applied be
bases of the other two transistors, one in each pair, of
tween the other terminal of said resistor and the collectors
> opposite conductivity type.
of the transistors'of other conductivity‘ type; means for
applying one input signal to the base of a transistor of 40
,No references cited.
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